1. Field of the Invention
The present invention relates to a laser beam emitting device and an optical pickup provided with the laser beam emitting device, and in particular relates to a laser beam emitting device for use in an optical pickup for reading out information from or writing information onto an optical disc and an optical pickup provided with the laser beam emitting device.
2. Description of the Background Art
In optical disc drives for optical discs such as CD-ROM or the like, an optical pickup is provided to read out information recorded on an optical disc. The optical pickup is equipped with a laser beam source for emitting a laser beam and an optical system for guiding the laser beam toward the optical disc.
Further, in recent years, disc drives in which a single optical pickup is used not only to read out information recorded on an optical disc but also to write information onto an optical disc have been developed (e.g., CD-R drives).
In the optical pickup having such reading and writing functions, the output of the laser beam source changes depending on whether information is being read out or written in. Specifically, in writing information onto the optical disc, a number of pits are formed onto an information recording surface of the optical disc by means of the emission of the laser beam. Therefore, when information is being written onto the optical disc, the laser beam output is made to be much larger (e.g., ten to twenty times larger) than the output used during reading out the information.
Now, in such an optical pickup, in order to change the intensity of the laser beam (quantity of light) emitted from the laser beam source, there is provided a means for monitoring (detecting) the intensity of the laser beam.
As shown in FIG. 4, such a means is equipped with a light-receiving element (PD: photodiode) 21 provided outside an effective optical path region 26 of a laser beam emitted by a laser beam source 20, in which laser light 28 outside the effective optical path region 26 is received by the light-receiving element 21 and photoelectrically converted into electrical signals. Then, based on the relative strength of thus obtained electrical signals, the intensity of the laser beam (quantity of light) emitted from the laser beam source 20 is detected.
However, even in the case where the output (the intensity of the emitted laser beam) of the laser beam source 20 is fixed, dispersions or deviations will arise in the electrical current value outputted by the light-receiving element 21 due to the reasons (1)-(3) listed below, and this makes it impossible to accurately monitor the intensity of the laser beam emitted from the laser beam source 20.
(1) Light-receiving elements have different individual characteristics (e.g., sensitivity).
(2) The illuminance distribution (far field pattern) of the emitted laser beam is different for each individual laser beam source.
(3) The intensity of the laser beam has a prescribed distribution (Gaussian distribution) depending on the distance from the optical axis 25, and for this reason there will be a difference in the quantity of light received by the light-receiving element 21 depending on the position of the light-receiving element 21, in particular the distance from the optical axis 25.
The problem described above becomes highly prominent in the case of the disc drives such as CD-R drives in which the output of the laser beam source undergoes a large change between the reading out and writing in of information.